1. Field of Industrial Utility
The invention relates to an optical sensor which can function such as a voltage sensor or a current sensor that can be used as an optical PT (potential transformer), an optical CT (current transformer), or the like for detecting the voltage to ground, the line current, or the like of a distribution line.
2. Related Art of the Invention
Conventionally, for example, a known voltage sensor of the intensity modulation type has the following configuration. As shown in FIG. 8, a first polarizer 47 having a set angle of .pi./2 radians, a quarter-wave plate 48, a Pockels device 49, and a second polarizer 52 having a set angle of .pi./2 radians are arranged on an optical axis in this sequence from the light entrance side. A voltage to be measured 51 is applied to the Pockels device 49. The reference numeral 50 designates electrodes of the Pockels device 49.
Nonpolarized incident light 53 is converted into linearly polarized light 54 by the first polarizer 47, and further converted into circularly polarized light 55 by the quarter-wave plate 48. The reference numeral 56 designates a polarization state of light which is changed by the voltage to be measured 51 applied to the Pockels device 49 and output therefrom. The change in intensity of light 57 output from the second polarizer 52 corresponds to the polarization state 56 of output light of the Pockels device 49 which changes depending on the voltage to be measured 51. Consequently, the voltage to be measured 51 can be detected by monitoring the modulation depth (degree of modulation) of the intensity of output light of the second polarizer 52.
The modulation depth (degree of modulation) of the intensity of light means a ratio of the AC component of the intensity of light and the DC component of the intensity of light.
For example, a prior art current sensor of the intensity modulation type has the following configuration. As shown in FIG. 9, a first polarizer 58 having a set angle of .pi./2 radians, a Faraday device 60, and a second polarizer 59 having a set angle of .pi./4 radians are arranged on the optical axis in this sequence from the light entrance side. A magnetic field 65 generated by the current to be measured is applied to the Faraday device 60.
Nonpolarized incident light 53 is converted into linearly polarized light 54 by the first polarizer 58, and further converted by the Faraday device 60 into linearly polarized light 62 which is changed by Faraday rotation angle (.theta.) 61 in accordance with the magnetic field 65 to be measured. The magnetic field 65 to be measured is a physical dimension corresponding to the current to be measured. The change in intensity of output light 63 of the second polarizer 59 corresponds to the polarization state (a change in polarizing angle of the linearly polarized light) of output light 62 of the Faraday device 60 which changes depending on the magnetic field 65. Consequently, the current to be measured can be detected by monitoring the modulation depth (degree of modulation) of the intensity of output light 63 of second polarizer 59.
However in either of the above-described optical sensors, in addition to an intensity changing which corresponds to the changing of the voltage or current to be measured a slow change in intensity of light is produced in said light output. Thus the AC component of the intensity of light output is distorted, because of the reason such as a change in coupling loss of the optical components, or a change in intensity of incident light which is caused by the external environment (for example, cloud or condensation on glass, displacement of the optical axis due to vibration, or generation of stress caused by a shake of an optical fiber). When the frequency of the voltage or current to be measured in the vicinity of DC, particularly, it is impossible to distinguish the AC component due to the voltage or current to be measured from that due to a physical dimension other than the voltage or current to be measured, resulting in that the voltage or current cannot correctly be measured. This is caused by the fact that both the optical sensors conduct AM modulation (amplitude modulation).